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Show I I . 202 DESCENT OF VAPOUR. the surface, and mounts up through the rest of the mass. If the air were perfectly st.ill (which it ne~er is), and the dense smoke of a furnace rose up w1th perfect uniformity (which that also never does), we should see the hyperbolic column monntii_Ig and swelling till the upper part of it became so t~m as to be invisible, and it seemed to melt away mto the air, hoth upward and laterally, with the m~st _finely melting shade imaginable. _And eve~ ~s It 1s, . al· thourrh the smoke is always uregular m 1ts quantity, and though those very irregularities produce l~ttle currents in the air, which throw the smoke mto curling volumes, an eye well disciplined in th~ ~b· servation of forms can trace the hyperbola m 1ts general outline, even when it is blown aside by a pretty smart breeze. The side opposite the wind is always more bent than the windward side, so that the column broadens as it gets distant from the chimney; and we hav_e only_ ~o imagine it to be raised straight and the mequahtles of the end~ to be arranged, in order to have a very. clear nohon of what it would be if the causes of dtsturbance were removed. The descent of a cloud, of a column of cold air, or of any thing else that can be so disper~ed through the atmosphere, is just the reve~s~ of tts asc~nt ; and therefore its form, if it were visible and undisturbed, would be hyperbolic, only with 3: downward motion, in place of an upward one, as m the fo!mer case. The motion would, however, be more rapid; and for that reason the descendino- hyperbolic mass would converge, or come togeth;r, more rapi~ly than the ascending one spreads. It ts ~rue that as 1t descended it would meet with more resistance from the denser a1r; and also from the upward current of air and o_f beat from the earth's surface, if the place under 1t happened to be wa~m ;. but still t~e ~eight of . the descending matter 1tself, the vel?ctty 1t had a~qmred in de~cending, and the attraction of coheswn be· ... ~..., ~ ' GttAVlTATtON ANl) COl:IESION. 203 twe~n its own particles, or portions, would all act Jh favour of the descent and the convero-ence whereas they act in opposition to the ascent ~nd tl~e spread Yf ere that n?t the case, we ~hould ha\'e water rising Jn showers, JUSt as often as 1t falls in showers, or, to speak more correctly, there would neither be the one nor the other; oecause, wherever it happened to be, the water would remain quite stationary. To understand well how nature works, it is absolutely necessary to have clear and perfect views of what. may be .called he~ elementary working, that workmg.m ~~tch there 1s no organization of parts, and n? ~nd1~1dual substance which we can in any way dtstmgmsh and o~serve. That, though it is not the first we co.me to, masmu?h ~s it is not apparent to the senses, 1s the true begmnmg of observation· and unless we comprehend it, we lose the greate; par~ both of the pleasure and the profit of the observatwn of individual things. The c~use of d~sc~nt is gravitation; the cause of aggregatwn, or brmgmg together or condensation of any kind, is cohesi_on; and.t~~ only force which ~e know that can act n~ opposttlon to, or overcome, either or b?th of these, 1s heat. Gravitation is, as it w~re, ~he he of all matter, without reference to any thmg .1!1 particular . kin~s of matter, but just their quantities. Coheswn 1s the particular tie which holds together t?e several kinds of matter, and it is perhaps the ultimate foundation of all their differ. ences. Th~ motion .of heat overcomes gravitation ouly by loostng coheswn,-by so dispersing the parts of a substance as .that they shall rise upward through a substance, specifically bghter than that which their form was before they were dispersed. Thus when ~eat acts so as to expand, and thereby to eievate, tt has always two resistances to contend with· 'Yhereas, when heat is diminishing, and concentra~ h~n and d?scent are taking place, these two act lomtly agamst the heat; and both of thetn act with |
